Spectrasonics Imaging, in collaboration with Riverside Research Institute (RRI) and Columbia Presbyterian Hospital proposes a Phase I effort to design, acquire and evaluate a linear array high intensity focused ultrasound (HIFU) transducer for the application of epicardial ablation for treating atrial fibrillation (AF). The transducer is to have both HIFU and pulse echo imaging capability - the latter for monitoring tissue change during the ablation and thereby controlling the process. HIFU has advantages over other ablation energy sources (e.g. RF) because the energy can be focused to produce homogeneous transmural lesions. The long term objective is to develop a system that will safely and effectively ablate the left atrium in a pattern that will block conduction from aberrant foci causing AF. The initial patient population will be AF patients undergoing cardiac surgery such as coronary artery bypass grafts (CABG) and mitral valve replacement (MVR). Eventually it may be possible to conduct the procedure thoroscopically and treat a much wider AF population. The Phase 1 program will involve determining frequency, size, focal and intensity requirements, devising a practical transducer design, and then modeling the design to verify beam shape and the resulting tissue temperature and damage.